Various metal oxides have found use in chemically reactive systems as catalysts, supports for catalysts, gettering agents and the like. As used herein, a gettering agent, or getter, is a substance that sorbs or chemically binds with a deleterious or unwanted impurity, such as sulfur. In those usages, their chemical characteristics and morphologies may be important, as well as their ease and economy of manufacture. One area of usage that is of particular interest is in fuel processing systems. Fuel processing systems catalytically convert hydrocarbons into hydrogen-rich fuel streams by reaction with water and oxygen. The conversion of carbon monoxide and water into carbon dioxide and hydrogen through the water gas shift (WGS) reaction is an essential step in these systems. Preferential oxidation (PROX) of the WGS product using such catalysts may also be part of the process, as in providing hydrogen fuel for a fuel cell. Industrially, copper-zinc oxide catalysts, often containing alumina and other products, are effective low temperature shift catalysts. These catalysts are less desirable for use in fuel processing systems because they require careful reductive activation and can be irreversibly damaged by air after activation.
Recent studies of automotive exhaust gas “three-way” catalysts (TWC) have described the effectiveness of a component of such catalysts, that being noble metal on cerium oxide, or “ceria” (CeO2), for the water gas shift reaction because of its particular oxygen storing capacity (OSC). Indeed, the ceria may even act as a “co-catalyst” with the noble metal loading in that it, under reducing conditions, acts in concert with the noble metal, providing oxygen from the CeO2 lattice to the noble metal surface to oxidize carbon monoxide and/or hydrocarbons adsorbed and activated on the surface. In many cases the ceria component of these catalysts is not pure ceria, but cerium oxide mixed with zirconium oxide and optionally, other oxides such as rare earth oxides. It has been determined that the reduction/oxidation (redox) behavior of the cerium oxide is enhanced by the presence of ZrO2 and/or selected dopants. Robustness at high temperatures is an essential property of TWC's, and thus, such catalysts do not typically have either sustainable high surface areas, i.e., greater than 100 m2/g, or high metal dispersion (very small metal crystallites), even though such features are generally recognized as desirable in other, lower temperature, catalytic applications.
For mixed-metal oxides that are to be used as such catalyst supports and which comprise cerium oxide and zirconium and/or hafnium oxide, it is generally desirable that they possess a cubic structure. The cubic structure is generally associated with greater oxygen mobility, and therefore greater catalytic activity. Moreover, the zirconium and/or hafnium provide thermal stability, and thus contribute to the thermal stability and life of a catalyst. Yashima et al., in an article entitled “Diffusionless Tetragonal-Cubic Transformation Temperature in Zirconia Solid Solution” in Journal of American Ceramic Society, 76 [11], 1993, pages 2865–2868, have recently shown that cubic ceria undergoes a phase transition to tetragonal when doping levels of zirconia are at or above 20 atomic percent. They suggest that above 20 percent zirconia, the oxygen anion lattice distorts into a tetragonal phase, while the cerium and zirconium cations remain in a cubic lattice structure, creating a non-cubic, metastable, pseudo-tetragonal phase lattice. Traditionally, powder X-ray diffraction (PXRD) is used to identify the structure and symmetry of such phases. However, in the case of ceria-zirconia oxides with very small crystallite sizes (i.e., less than 3 nm), the PXRD signal exhibits broadened peaks. Additionally, the signal produced by the oxygen atoms, which is a function of atomic weight, is drowned out by the intense signal produced by the cerium and zirconium cations. Thus any tetragonal distortion, caused by the oxygen atoms shifting in the lattice, goes unnoticed in a PXRD pattern and the resulting pattern appears cubic. In such cases, Raman spectroscopy and X-ray absorption fine structure (EXAFS) can be employed to observe such phase transitions. Yashima et al. have published Raman spectroscopy and EXAFS studies in support of the position taken above. Vlaic et al., in an article entitled “Relationship between the Zirconia-Promoted Reduction in the Rh-Loaded Ce0.5Zr0.5O2 Mixed Oxide and the Zr—O Local Structure” in Journal of Catalysis, 168, (1997) pages 386–392, have shown similar results for a phase transition at 50% zirconia, as determined by Raman spectroscopy and EXAFS.
Ceria-zirconia mixed oxide materials having relatively large surface areas per unit weight may be particularly well suited in various catalytic and/or gettering (i.e., sulfur sorbing) applications, as might be typified by, but not limited to, the WGS reaction. Indeed, such ceria-based mixed metal oxides may be used first in a WGS system as a getter to adsorb sulfur-containing compounds from the gas stream to protect more sensitive/valuable components downstream that use such oxides as catalysts in the WGS reaction. In that general regard, it is deemed desirable that the mixed oxide material be comprised of small crystallites agglomerated to form porous particles having relatively large surface areas per unit weight as a result of significant pore diameters and pore volumes. Large pore diameters facilitate mass transfer during catalytic reactions or gettering applications, by minimizing mass transfer resistance. On the other hand, excessive pore volumes may act to minimize the amount of effective surface area in a given reactor volume, for a given final form of catalyst or getter, thereby limiting the catalytic or gettering action in a given reactor volume. Thus, the ratio of pore volume to the structural mass, as well as crystallite size and pore diameters, can be optimized within a range. In this regard then, the particular morphology of the ceria-based mixed-metal oxide material becomes important for efficient operation of the material as a catalyst or getter in particular reactions and/or under particular operating conditions and geometries.
A variety of synthesis techniques have been used to provide ceria-zirconia mixed oxide materials. These techniques include conventional co-precipitation, homogeneous coprecipitation, the citrate process, and a variety of sol-gel techniques. However, as far as can be determined, the surface areas of the mixed metal oxides resulting from these techniques are typically less than about 130 m2/g. Liquid phase synthesis at relatively low temperatures is preferred, as it allows for the formation of metastable phases and offers the ability to control such properties as surface area, particle size, and pore structure. Typical solution routes have involved two steps, hydration and condensation. It has been generally accepted that the gel matrix formed upon hydration is amorphous and only forms a crystalline structure when the framework undergoes condensation. While hydration occurs at the moment the gelatinous phase is formed from solution, condensation has usually been expected to occur during the aging (maturing), drying and/or calcinations steps. For many mixed metal oxide systems, the detailed conditions under which these steps (such as aging) occur are, and have been, critical parameters in determining the properties of the final product. Thus, a time consuming step such as aging has been essential.
Surface areas as great as 235 m2/g for such materials have been reported by D Terribile, et al, in an article entitled “The preparation of high surface area CeO2—ZrO2 mixed oxides by a surfactant-assisted approach” appearing in Catalysis Today 43 (1998) at pages 79–88, however, the process for their production is complex, sensitive, and time-consuming. The process for making these oxides requires the use of a surfactant and a lengthy aging, or maturing, interval of about 90 hours at 90° C. Moreover, the initial precipitate must be washed repeatedly with water and acetone to remove the free surfactant (cetyltrimethylammonium bromide) before the material can be calcined, thereby contributing to delays and possible other concerns. Still further, the mean particle sizes of these oxides appear to be at least 4–6 nm or more. The pore volume is stated to be about 0.66 cm3/g. This relatively large pore volume per gram is not consistent with that required for a ceria-based mixed metal oxide which, while thermally robust, should tend to maximize both the available surface area in a given reactor volume and the mass transfer characteristics of the overall structure as well as the appropriate reactivity of that surface area, as is desired in the applications under consideration. Assuming the density, D, of this material is about 6.64 g/cm3 the skeleton has a volume, VS, of 1/D, or about 0.15 cm3/g, such that the total volume, VT, of one gram of this material is the sum of the pore volume, PV, (0.66 cm3/gm) and the skeletal volume, VS, which equals about 0.81 cm3/gm. Hence, 235 m2/gm÷0.81 cm3/gm equals about 290 m2/cm3. Because of the relatively large pore volume, the surface area per unit volume of a material of such density has a reduced value that may not be viewed as optimal.
For use of a mixed-metal oxide in a catalyst application, it is required to be loaded with a metal, such as a noble metal, providing good catalytic activity to the media being processed. While noble metals such as platinum have provided good catalytic activity, it is always desirable to improve the activity, cost, and/or durability of such catalyst metal loadings.
It is desirable to provide ceria-based mixed-metal oxide materials having the aforementioned positive properties and avoid the limitations, for use in catalytic reactions/gettering applications generally, and fuel processing catalytic reactions/gettering applications more specifically. Even more particularly, it is desirable to provide such ceria-based mixed-metal oxide materials for use in, for example, water gas shift reactions employed in fuel processing systems for the production of hydrogen-rich feed stock.
Accordingly, it is an object of the invention to provide ceria-based mixed metal oxides having the aforementioned desirable properties of relative stability, high surface areas, relatively small crystallites, and pore volumes sized to optimally balance the reduction of mass transfer resistance with the provision of sufficiently effective surface areas in a given reactor volume, particularly for use as a catalyst support or co-catalyst, though not limited thereto.
It is another object of the invention to provide such ceria-based mixed-metal oxides having enhanced redox capability, and moreover possessing good thermal stability.
It is an even further object of the invention to provide a catalyst including the ceria-based mixed metal-oxide as a support, in accordance with the forgoing objects. Further to this object, it is desired to provide the support with a catalyst metal loading that exhibits enhanced activity, cost, and/or durability.
It is a still further object of the invention to provide an efficient and economical process for making such ceria-based mixed-metal oxide catalyst supports, catalysts, and/or getters in accordance with the foregoing objects.
It is yet a further object of the invention to utilize a catalyst employing a ceria-based mixed metal oxide as a support, co-catalyst, or getter, made in accordance with the foregoing objects, in a fuel processing system in, for example, a water gas shift reaction.